1. Field of the Invention
This invention relates in general to a transflective liquid crystal display (LCD). More particularly, this invention relates to a transflective liquid crystal display (LCD) with a single gap type and capable of balancing a color difference between the reflective and transmissive regions.
2. Description of the Related Art
Due to the characteristics of thin profile and low power consumption, liquid crystal displays (LCDs) have been widely used in electronic products, such as portable personal computers, digital cameras, projectors, etc.
Unlike CRTs (cathode ray tubes) and EL (electroluminescent) devices, LCDs do not emit light themselves, hence, transmissive LCDs require a backlight module. The backlight usually consumes 50% or more of the total power consumed by the LCD device. Therefore, power consumption is increased due to the use of backlight.
For solving the aforementioned problem, a reflective type LCD is thus developed for people who rely primarily on an LCD device or use the LCD device outdoors. A reflective type LCD comprises a reflector, instead of a backlight, to reflect ambient light. Reflective LCD devices, generally, include TN (twisted nematic) mode devices and STN (super twisted nematic) mode devices.
The reflective type LCD has the drawback of low visibility when ambient light decreases. In contrast, the drawback of the transmissive type LCD is that the visibility is very low when the ambient light is very bright. Since the display is darker than the ambient light, the quality of color reproduction suffers. In order to improve the image quality of the display in a brightly lit environment, it is necessary to enhance the light intensity of the backlight. However, enhancing the light intensity of the backlight increases power consumption. Moreover, when the LCD is exposed to sunlight or another light source, the image quality of the display decreases considerably. For example, when the sunlight or another light source is directly incident to the LCD monitor, it is difficult to view the displayed image due to reflection.
In order to solve the above problems, both the transmissive type display and the reflective type display are constructed as a single LCD, which is a so-called transflective type LCD. FIG. 1 is a top view of an active array substrate of a transmissive type LCD, disclosed in U.S. Pat. No. 6,295,109 and issued to Sharp Corp. FIG. 2 is a cross section cut along the line II–II′ of the transmissive type LCD in FIG. 1.
As shown in FIGS. 1 and 2, gate lines 53 and source lines 59a are perpendicular to each other and arranged on a transparent substrate. Thin film transistors (TFTs) 57 are respectively disposed in the vicinity of the intersection of each gate line 53 and each source line 59a. The drain electrode 59c of the TFT 57 is connected to a transmissive electrode 58a, used as a portion of a pixel electrode, for providing voltage to the liquid crystal material in the transmissive region T. In the reflective region R, an interlayer insulating layer 60 and a reflective electrode 61 are disposed on the transmissive electrode 58a. The reflective electrode 61 is connected to the drain electrode 59c via a contact hole 63, and used as a portion of the pixel electrode. Therefore, when viewing the above described LCD, the transmissive region T possesses a considerably high light transmission ratio, while the reflective region R possesses a considerably high light reflection ratio.
As shown, the TFT 57 comprises a gate insulating layer 54, a semiconductor layer 55, semiconductor contact layers 56a and 56b, a source electrode 59b, a drain electrode 59c and a gate electrode 52. The gate electrode 52 is branched from the gate line 53.
The light in the transmissive region T will pass through the liquid crystal layer with a thickness of dt once, while the light in the reflective region R will pass through the liquid crystal layer with a thickness of dr twice. In order to match retardation caused by the liquid crystal layer in the reflective region R and retardation caused by the liquid crystal layer in the transmissive region T, a thickness relationship of dt=2×dr must be satisfied. The thickness dr of the liquid crystal layer in the reflective region R and the thickness dt of the liquid crystal layer in the transmissive region T can be adjusted by varying the thickness of the interlayer insulating layer 60.
The aforementioned transmissive type LCD has two gaps, i.e., dt and dr. Although the color difference between the reflective region R and the transmissive region T is almost the same, a height difference between the reflective region R and the transmissive region T hinders formation of an alignment film by rubbing during the manufacturing process.